Device for discontinuing and automatically restoring the operational function of a spring brake actuator

ABSTRACT

A manual disconnecting device is provided for disengaging the brake push rod in a spring brake actuator having a movable piston holding the spring compressed by fluid pressure during normal operation. The device once manually disconnected is automatically reconnected upon a normal spring compression stroke of the piston. The disconnecting device provides a clutching device between the piston and the push rod engaging a nut threaded on the push rod with non-self-locking threads. In one embodiment the nut us journalled in the piston. In another embodiment there are clutch surfaces on both the piston and nut which are disengaged by a rotary motion from a special tool.

This is a continuation of application Ser. No. 514,929, filed Oct. 15,1974, now abandoned.

A conventional spring brake actuator comprises a cylinder, a piston, anda spring actuating the piston. The spring is normally held compressed byfluid pressure acting on the piston, and at a pressure fall the springwill force the piston, which is connected to further parts of a vehiclebrake rigging, in a brake applying direction. Such an actuator isnormally used as a parking and emergency brake actuation for a vehicleand is in such a case attached to a service brake actuator, whose pistonwill be actuated by a push rod of the spring brake actuator.

If the vehicle has to be moved in the absence of fluid pressure for thespring brake actuator it is necessary to relieve the force from thespring on the brake rigging. One way to accomplish this is to compressthe spring manually by means of an external tool engaging the piston orthe push rod. This work is however both tiresome and time-consuming.Another and in most cases preferred way is to disengage the push rodfrom the piston, so that the push rod can be retracted. A requirement isthat the normal function of the actuator shall be restored automaticallyafter the return of fluid pressure for the spring compression.

This invention thus relates to a device for discontinuing andautomatically restoring the operational function of a spring brakeactuator, preferably arranged in series with a service brake actuatorand comprising a cylinder, a piston, a push rod, a spring actuating thepiston in an operation direction forward for brake application at thefall of a fluid pressure acting on the other side of the piston, and aconveying mechanism between the piston and the push rod, which mechanismat the piston movement forward is arranged to convey the push rod but isdisconnectable for making possible a return of the push rod alone at acarried forward piston and is arranged to return automatically to aposition for again conveying the push rod after the piston return.

Such devices are known but are marred by certain disadvantages, such asbeing complex, expensive or, last but not least, less reliable.

The object of the present invention is to accomplish a device of thekind referred to which is extremely simple and cheap but still veryreliable and easy to control manually.

This is according to the invention attained in that the conveyingmechanism consists of a nut in non-self-locking engagement withscrew-threads on the push rod and an externally disengagable,spring-actuated clutch between the nut and the piston, the clutch beingarranged automatically to be re-engaged under the influence of theforces appearing at the piston return.

This means that the clutch will be held in continuous engagement duringall normal operation of the spring brake actuator and thus that no partsof the conveying mechanism move relative to each other under normalconditions, making the device very reliable. The clutch is onlymechanically disengaged at the disconnection of the piston and the pushrod, so that the latter may be returned to its initial position underthe influence of the return spring in the brake rigging, especially thereturn spring in the service brake actuator.

It is to be noted that the spring brake actuator need not be connectedto a service brake actuator. In many cases it is preferred to dispersewith the service brake actuator and to use the spring brake actuatoralso for service braking. In such cases there may be springs in thebrake rigging for the return of the piston rod, or else the push rod maybe retracted manually after the disconnection of the piston and the pushrod.

In a first embodiment of the invention the nut is rotatably journalledin the piston and a manually actuatable control arm is attached to thepiston in resilient engagement with ratchet teeth on the nut. The pistonrod is in this case provided with a flange for cooperation with thepiston during the piston return.

In a second embodiment there are cooperating clutch surfaces, preferablyprovided with teeth, on the nut and the piston. These surfaces are heldin engagement with each other by means of a clutch spring between aflange on the piston and an anti-friction bearing on the nut, and thereis an externally actuatable sleeve for disengagement of the clutchsurfaces against the action of the clutch spring. This sleeve is innon-self-locking thread engagement with the piston, and an anti-frictionbearing is arranged on the sleeve for engagement with the nut. By thisarrangement it is possible to disengage the clutch by turning the sleevewith an appropriate tool, and the sleeve will return to its initialposition due to its non-self-locking engagement with the piston.

The invention shall be described in further detail below, referencebeing made to the accompanying drawings, in which:

FIGS. 1-3 are sectional views of a first embodiment of the invention inthree different working positions, and

FIG. 4 is a sectional view of a second embodiment of the invention.

As appears from FIG. 1 a spring brake cylinder 1 is attached to aservice brake cylinder 2. A spring brake piston 3 is acted upon by onone side a spring 4 and on the other side fluid pressure in a fluidchamber 5. The central part of this piston can be called a piston rod 3Aand is guided for axial movements by an opening in the service brakecylinder end wall and a guiding tube 6 attached to the spring brakecylinder 1 and having a closed end constituting a stop for the returnmovement (to the right) of a push rod 7. This push rod is provided witha flange 7A as shown and is arranged to act on a service brake piston 8,which as usual is connected to suitable parts of the brake rigging andis acted upon by a return spring 8A.

A nut 9 is rotatably journalled in the piston 3 by means ofanti-friction bearings 10 and has a non-self-locking engagement withthreads on the push rod 7. The nut is provided with ratchet teeth on itsouter periphery. A control lever 11 is pivotally attached to the piston3 and extends out from the cylinder 1. The lever is urged intoengagement with the ratchet teeth on the nut 9 as shown in FIG. 1 bymeans of a spring (not shown), thus constituting a spring-actuatedclutch 12.

The thread direction on the push rod 7 and the locking direction ofthese ratchet teeth in cooperation with the control lever 11 are suchthat on normal application at falling fluid pressure and under theinfluence of the spring 4 the nut 9 will be locked on the push rod 7,which means that the force from the piston will be transmitted to thepush rod 7 via the nut 9 (and then further to the brake rigging via theservice brake piston 8). On the return stroke (under the influence offluid pressure in the fluid chamber 5) the force from the piston will betransmitted to the push rod 7 over its flange 7A.

The conditions at the loss of fluid pressure in the fluid chamber 5 areshown in FIGS. 2 and 3, which are not provided with any referencenumerals for the sake of clarity. Thus, if the brakes are applied and nofluid pressure is available for the desired return of the piston 3, thecontrol lever 11 is to be lifted to the position shown with dash-dottedlines in FIG. 2. The different parts will then assume the positionsshown in FIG. 3, i.e. the push rod 7 will move to its extreme rightposition under the influence of the return spring 8A for the servicebrake piston 8, whereas the spring brake piston 3 will move to itsextreme left position under the influence of the spring 4 (and thecontrol lever 11 will be spring-biassed to the shown position engagingthe ratchet teeth on the nut 9).

It is now evident that the spring 4 has no influence on the servicebrake piston and thus not on the vehicle brake rigging and that it willbe possible to move the vehicle in spite of the fact that no fluidpressure is availabe, i.e. that the spring brake actuator is applied.

When the fluid pressure returns to the fluid chamber 5 the piston 3 willmove to the right, and during this movement the nut 9 will rotate on thepush rod 7 and the control lever 11 will only slide on the ratchet teethdue to their opposite locking direction. When the position according toFIG. 1 has been reassumed the spring brake actuator is ready for normaloperation.

In the second embodiment according to FIG. 4 a spring brake cylinder 20is attached to a service brake cylinder 21 which is provided with aservice brake piston 22 and a piston return spring 22A. A spring brakepiston 23 is acted upon by on one side a spring 24 and on the other sidefluid pressure in a fluid chamber 25. The central part of the piston isguided for axial movements by suitable tubular parts of the twocylinders and can be called a piston rod 23A. The force from the piston23 to the service brake piston 22 and further to parts of the vehiclebrake rigging is trnsmitted over a push rod 26 via a nut 27 innon-self-locking engagement with screw threads on the former. Betweenthe piston rod 23A and the nut 27 there is a dog clutch 28 as shown,which is normally held in engagement by a clutch spring 29 acting on thenut 27 via an anti-friction bearing 30. The push rod 26 is provided withan end plate 26A acting as a stop for the return movement.

The device is further provided with a control sleeve 31 threadinglyengaging the piston rod 23A as shown. This engagement is preferablynon-self-locking. There is a positive stop for the return movement ofthe control sleeve. An anti-friction bearing 32 is mounted on thecontrol sleeve 31 at some distance from the nut 27. The control sleeveis provided with attachments for a tool 33.

At a fluid pressure fall in the fluid chamber 25 the spring 24 willexert its force on the piston 23, the piston rod 23A, the nut 27, andthe push rod 26, and further on suitable parts of the vehicle brakerigging (not shown). At the return of the fluid pressure the differentparts will return to their shown positions; the dog clutch 28 will notbe disengaged under this normal operation.

If the force from the spring 24 on the push rod 26 is to be discontinuedat the absence of fluid pressure in the fluid chamber 25, the centralsleeve 31 (or rather its anti-friction bearing 32) is brought intoengagement with the nut 27 by turning the sleeve by means of the tool33, so that the dog clutch 28 is disengaged. The nut 27 will now be freeto rotate on the push rod 26, which will return to the shown positionunder the influence of the return spring 22A in the service brakeactuator, whereas the piston 23 will move to its extreme left positionunder influence of the spring 24. When the fluid pressure returns thepush rod 26 will be prevented from moving to the right due to the plate26A, whereas the dog clutch 28 will be disengaged due to the force nowacting to the right on the piston rod 23A and the stopping force on thepush rod 26. The effect will be that at the end the different parts willhave the shown positions and that the device will be ready for normaloperation.

It is to be noted that the invention is not limited to spring brakeactuators attached to service brake actuators and used as parking andemergency actuators. Spring brake actuators are sometimes used alone, inwhich case the fluid pressure will govern the force from the spring onthe piston rod. The invention is equally applicable to such actuators,provided that there is any means for returning the push rods to theirinitial positions, for example a return spring in the brake rigging.

Modifications are possible within the scope of the appended claims.

We claim:
 1. In a device for discontinuing and automatically restoringthe operational function of a spring brake actuator with a springcompressed by fluid pressure in the brake system, preferably arranged inseries with a service brake actuator and comprising a cylinder with afluid pressure operated piston movable therein to operate a brake pushrod and having a spring actuating the piston in an operation directionforward for brake application at the fall of fluid pressure acting onthe piston, the combination comprising, a conveying mechanism carried bythe push rod, which mechanism at the piston movement forward is arrangedto convey the push rod but is disconnectable for making possible areturn of the push rod alone when said fluid pressure falls and isarranged to return automatically to a position for again conveying thepush rod after the fluid pressure returns, wherein the conveyingmechanism consists of a nut in non-self-locking engagement withscrew-threads on the push rod, a clutch movable with the piston forholding the nut from rotation, manually operable means extendingexternal to said piston for disengaging the clutch until reengaged bymovement of said piston when the fluid pressure returns the piston toits return position with brakes disengaged to thereby permit the nut torotate in response to relative movement between said piston and saidpush rod thereby comprising means mechanically connected to said pistoncausing return movement by said piston automatically to rotate said nutwith said clutch in disengaged position and including structurere-engaging said clutch to convey said push rod by rotating said nutunder the influence of the forces transmitted by the moving piston toengage the clutch for a braking stroke when the fluid pressure movessaid piston to the end of its return stroke said clutch comprisingratchet teeth on said nut and means engaging them, the nut beingrotatably journalled in the piston and said means for disengaging theclutch comprises a control arm pivoted on the piston and held inengagement actuating the clutch and constituting a pawl mating with saidratchet teeth on the nut thereby registering with said control arm toengage said clutch, and having a manually accessible member for pivotingthe control arm to release said clutch and permit said nut to rotate onsaid rod while out of engagement with said control arm in a directionotherwise held from rotation by said control arm.
 2. A device accordingto claim 1, characterized by a flange on the push rod for engagementwith the piston during the piston return to move said push rod androtate thereon said nut into position on said push rod engaging saidcontrol arm for permitting the piston to operate said brake push rod inthe forward direction.